Coherent light illuminating a rough surface produces speckle. Reflection from the rough surface is referred to as diffuse reflection. Transmission through the rough surface is referred to as diffuse transmission. In the diffuse reflection or the diffuse transmission, light scatters in various directions. The coherent light scattered by the diffuse reflection or by the diffuse transmission forms an interference pattern in the space away from the rough surface. If viewed by a human eye, the eye will see dark and light in a `granular` pattern. The granular pattern is the speckle. An intensity detector of an optical system will also detect the speckle if the optical system views the rough surface illuminated by the coherent light.
A speckle demonstration apparatus of the prior art is illustrated in FIG. 1. The speckle demonstration apparatus 1 includes a demonstration laser 2, a diverging lens 4, and a viewing screen 6, which are located on a first optic axis 8. The demonstration laser 2 emits a laser beam 10. The diverging lens 4 transforms the laser beam 10 into a divergent laser beam 12. The divergent laser beam 12 illuminates the viewing screen 6 in a large area 14. The viewing screen 6 diffusely reflects the divergent laser beam 12 creating an interference pattern. An observation plane 16 located on a second optic axis 18 intersects the interference pattern. The observation plane 16 is the field-of-view in space where the eye or the optical system is focused. If the eye or the optical system is focused at the viewing screen 6, the observation plane 16 is located at the viewing screen 6. Note that the diverging lens 4 aids in demonstrating the speckle but is not necessary to produce the speckle.
FIG. 2 is a photograph of a typical speckle pattern 17 of the prior art, which is illustrative of the speckle viewed at the observation plane 16. Constructive interference of the divergent laser beam 12 reflecting diffusely from the viewing screen 6 creates bright spots in the observation plane 16. Destructive interference creates dark spots between the bright spots. The diffuse reflection from the viewing screen 6 has a random nature so the bright spots and the dark spots vary throughout the observation plane 16.
A measure of the speckle is contrast (C). The contrast, in percent, is given by C=100*I.sub.RMS /I where I is a mean intensity and I.sub.RMS is a root mean square intensity fluctuation about the mean intensity.
Goodman in "Some fundamental properties of speckle," J. Opt. Soc. A., Vol. 66, No. 11, November 1976, pp 1145-1150, teaches that the speckle can be reduced by superimposing N uncorrelated speckle patterns. This reduces the contrast by a speckle reduction factor of N provided that the N uncorrelated speckle patterns have equal mean intensities and contrasts. If the N uncorrelated speckle patterns have non-equal mean intensities or non-equal contrasts, the speckle reduction factor will be less than N. Thus, the speckle reduction factor of N is a best case for the speckle reduction for the N uncorrelated speckle patterns. Goodman further teaches that the uncorrelated speckle patterns can be obtained by means of time, space, frequency, or polarization.
A speckle reduction method of the prior art creates multiple speckle patterns by moving the viewing screen 6 in an oscillatory motion 19, which employs the time means taught by Goodman. The oscillatory motion 19 typically follows a small circle or a small ellipse about the optic axis 8. This causes the speckle pattern to shift relative to the eye or the optical system viewing the viewing screen 6 and, thus, forms multiple speckle patterns over time. Though the amount of the speckle at any instant in time is unchanged, the eye perceives the reduced speckle provided that the speed of the oscillatory motion is above a threshold speed. The intensity detector of the optical system detects the reduced speckle provided that an exposure time is sufficiently long to allow the speckle pattern to move a significant distance.
In the art of laser illuminated display systems, it is known that an active diffuser can be added to a laser illuminated imaging system for reducing laser speckle. The active diffuser is placed in an intermediary image plane or near the intermediary image plane. The active diffuser is moved in the intermediate image plane in a rotation or toroidal pattern about a display system optic axis in order to create a shifting phase at a display screen. The shifting phase creates uncorrelated speckle patterns over time, thus employing the time means, taught by Goodman.
Wang et al. in "Speckle reduction in laser projection systems by diffractive optical elements," Applied Optics, Vol. 37, No. 10, Apr. 1998, pp 1770-1775, teach a method of laser speckle reduction in a laser projection system such as a laser television system. In the laser projection system a laser spot forms an image on a display screen by a raster scan similarly to how an electron beam forms an image in a CRT (cathode ray tube) display. The method taught by Wang et al. is accomplished by expanding a laser beam, placing a diffractive optical element in the expanded laser beam to form multiple beamlets, and then focusing the laser beamlets to form the laser spot on the display screen. The multiple beamlets shift slightly as each pixel is formed on the display screen. This provides a time varying speckle pattern and consequently a speckle reduction. Wang et al. further teach that the diffractive optical element can be rotated to slightly improve the speckle reduction.
Bloom et al. in U.S. Pat. No. 5,982,553 issued on Nov. 9, 1999, incorporated herein by reference, teach a display system including a grating light valve (GLV), red, green, and blue lasers, various lens arrangements, a scanning mirror, a display screen, and electronics. The electronics control the GLV, the lasers, and the scanning mirror to form a two dimensional image on the display screen.
In the display system taught by Bloom et al., the GLV forms a line image composed of a linear array of pixels on the display screen. The scanning mirror repeatedly scans the line image across the display screen in a direction perpendicular to the line image as the GLV modulates the linear array of pixels thereby forming the two dimensional image.
Because the two dimensional image taught by Bloom et al. is formed by laser illumination, the two dimensional image exhibits laser speckle, which degrades an image quality. It would be desirable to improve the image quality by reducing the laser speckle.
What is needed is a method of reducing laser speckle in a display system where a two dimensional image is formed by scanning a line image.
What is needed is a method of reducing laser speckle in an optical system where a line illumination produced by a laser is scanned over a diffuse surface.